Cattle are monovular and produce only one calf per year in the natural state. For improving the reproductive efficiency of cow, effects have been conducted to control the number of dominant follicles in the ovary by using various methods, and induce to form codominant follicles and then ovulate two follicles, thereby achieving the purpose of producing two calves.
Currently, several methods are frequently used to induce a cow to produce two calves, which were showed as follow: (1) Hormone induction method: using hormones such as PMSG or FSH (TAN Niannian, 2000; WANG Zhengchen, 2004) to induce a cow to produce two calves through early-stage small-dosage treatment, however, the half-life of PMSG is long, and thus the development of follicles would be disturbed consistently, therefore the stability and feasibility of this method is poor; further, with respect to the method by treatment with FSH alone, the number of codominant follicles cannot be easily controlled, therefore it is not feasible and convenient for practical application. (2) Mechanical aspiration method: aspirating all of the follicles ≧4 mm 4-6 days after ovulation by virtue of a B ultrasound machine (Acosta et al., 2005; Palhao et al., 2009) so as to make the concentration of FSH decrease to the lowest level on the day of aspiration (Kulick et al., 2001), thereafter, the elevation of FSH and the emergence of new follicular wave are just synchronous, thus increasing the probability of producing codominant follicles. (3) Hormone plus mechanical aspiration method: a study (G. Glick et al., 2013) has used small doses of FSH to induce double ovulations of a cow. Although 2-3 ovulations can be achieved at the rate of 90%, the procedure thereof comprises using special estrus synchronization treatment 13 days before the experiment, and aspirating more than 3 large follicles via vaginal wall by using a B ultrasound machine on day 7 of the experiment, so as to achieve the purpose of 2-3 ovulations. Therefore, it is difficult to popularize in the production due to high technical operation and expensive instruments required by follicle aspiration. (4) Inhibitor immunization: some studies used inhibin (gene) immunization (YANG Liguo, 2002; WANG Shuilian, 2009) to adjust the endogenous secretion of FSH, and achieved some effects, but the extraction and the purification of inhibin and the preparation of gene vaccine are complex, and commercial drugs are not commercialized yet, thus it is not popularized in the practice.
All above these methods have defects such as poor stabilization or difficult operational technology, and are difficult to be generalized and applied practically. Hence, there is a need to develop a simple and practical method for inducing twin calving in the field application.